FIG. 3 illustrates a cross-sectional view of a conventional semiconductor laser including a ridge waveguide (hereinafter) referred to as a ridge type semiconductor laser), taken parallel to facets of the laser resonator. In the figure, reference numeral 1 designates an n-GaAs substrate. An n-Al.sub.0.3 Ga.sub.0.7 As lower cladding layer 2, a quantum well active layer 3, ap-Al.sub.0.3 Ga.sub.0.7 As first upper cladding layer 4, a p-Al.sub.0.7 Ga.sub.0.3 As etch-stopping layer 5, a p-Al.sub.0.3 Ga.sub.0.7 As second upper cladding layer 6, and a p-GaAs contact layer 7 are disposed in this order on the substrate 1. A ridge waveguide 9 has a ridge structure extending like a stripe. A p side electrode 12 and an n side electrode 13 are disposed on top and bottom of the laser, respectively. An insulating film for current confinement 14 is disposed on the ridge waveguide 9 and the etch-stopping layer 5, having an opening 15 forming a stripe-like surface on the upper flat surface of the ridge waveguide 9.
FIGS. 4(a), 4(c), 4(d), and 4(e) are cross-sectional views and FIG. 4(b) is a perspective view, illustrating a method of fabricating the conventional ridge type semiconductor laser. In these figures, the same reference numerals as shown in FIG. 3 designate the identical or corresponding parts; reference numeral 8 designates an insulating film having a stripe-like surface.
FIG. 5 is a cross-sectional view illustrating a principal process of the method of fabricating the conventional ridge type semiconductor laser. In the figure, the same reference numerals as shown in FIGS. 4(a)-4(e) designate identical or corresponding parts.
A description is given of the method of fabricating the conventional ridge type semiconductor laser.
Initially, on the n-GaAs substrate 1 in a wafer state (the wafer state is not shown), the n-Al.sub.0.3 Ga.sub.0.7 As lower cladding layer 2, the quantum well active layer 3, the p-Al.sub.0.3 Ga.sub.0.7 As first upper cladding layer 4, the p-Al.sub.0.7 Ga.sub.0.3 As etch-stopping layer 5, the p-Al.sub.0.3 Ga.sub.0.7 As second upper cladding layer 6, and the p-GaAs contact layer 7 are epitaxially grown in this order. FIG. 4(a) shows a cross-sectional view of the wafer after completing the epitaxial growth.
Thereafter, the wafer is completely covered with an insulating film (not shown) and then coated with a photoresist(not shown) having the desired pattern which is formed by means of photolithographic techniques. Using the photoresist as a mask, the insulating film 8 is selectively etched to a shape like a stripe. Si.sub.3 N.sub.4, SiO.sub.2 or the like is used as the material for the insulating film. The stripe-like insulating film 8 serves as a mask for etching to produce a ridge waveguide. FIG. 4(b) shows a cross-sectional view after the patterning of the insulating film 8.
Thereafter, using the insulating film 8 as a mask, the p-GaAs contact layer 7 and the p-Al.sub.0.3 Ga.sub.0.7 As second upper cladding layer 6 are selectively etched, the etching being stopped at the p-Al.sub.0.7 Ga.sub.0.3 As etch-stopping layer, to produce a ridge waveguide having a ridge structure extending like a stripe in the desired direction. Examples of a solution for etching are a mixture of tartaric acid and hydrogen peroxide, or a mixture of sulfuric acid, hydrogen peroxide, and water. FIG. 4(c) shows a cross-sectional view after the etching step.
After the etching to produce a ridge structure, the stripe-like insulating film 8 is selectively removed by wet or dry etching. The entire wafer is covered with insulating film 14 again. Further, the photoresist 16 is deposited on the insulating film 14 and, by use of the photolithographic technique, an opening 17 is made in the photoresist on the upper flat surface of the ridge waveguide 9 shown in FIG. 4(e). Using the photoresist 16 as a mask, a portion of the insulating film 14 situated on the upper flat surface of the ridge waveguide 9 is selectively removed by dry etching or the like to form opening 15. Furthermore, the p side electrode 12 is formed on top of the wafer. Thus, the p side electrode 12 comes into contact with the contact layer 7 only through the opening 15. That is, the current is allowed to flow only through the opening 15.
Finally, the n side electrode 13 is formed on the rear surface of the substrate 1. The wafer is cleaved into chips, making facets of laser resonators, thereby completing the semiconductor laser shown in FIG. 3.
A description is given of how the conventional ridge-type semiconductor laser operates.
When voltage is applied across the p side electrode as a plus pole and the n side electrode as a minus pole, holes are injected into the active layer 3 through the p-GaAs contact layer 7, the p-AlGaAs second upper cladding layer 6 and the p-AlGaAs first upper cladding layer 4, while electrons are also injected into the active layer 3 through the n-GaAs substrate 1 and the n-AlGaAs lower cladding layer 2. The recombination of electrons and holes occurs in the active region of the quantum well active layer 3, producing light by the stimulated emission of radiation. Provided that a substantial volume of carriers are injected into the active region to produce light beyond the loss of the ridge waveguide, laser oscillation will occur.
At this time, current can not flow through the other regions except the upper flat surface of the ridge waveguide 9 since those regions are covered with the insulating film 14. Namely, the current is allowed to flow only into the ridge waveguide 9. Thus, the quantum well active layer 3 disposed under the ridge waveguide 9 serves as an active region so as to produce laser oscillation.
A semiconductor laser generally confines light within an active region due to a difference in the refractive indices between an active layer and cladding layers along a direction perpendicular to the surface of a substrate. Therefore, light confinement along a vertical direction is effective along the entire waveguide of the semiconductor laser. As opposed to this, a ridge type semiconductor laser guides light along a direction parallel to the surface of a substrate due to an effective difference in the refractive indices between a ridge waveguide and its adjacent regions. When the width of a ridge is constant, higher modes of oscillation occur more easily, the larger the difference in the refractive indices between the ridge waveguide and its adjacent regions. Conversely, as the difference in the refractive indices decreases, the tolerable width of the ridge without higher modes of oscillation becomes larger. Namely, it is possible for a wider ridge to cut off higher modes. In this case, however, as the volume of the injected current is increased, the refractive index decreases in the middle of the ridge where current density is high. Hence, a slight fluctuation of the current distribution occurs, which causes the phenomenon whereby the spot of light floats. As a result, a non-linear state, the so-called kink, emerges on the light output versus current characteristics in which light output is not proportional to current. A kink means that there will be a serious failure during the actual use of the device.
A solution to this problem is to make the ridge sufficiently narrow so that higher modes of oscillation do not occur. When the width of the ridge is less than 3 .mu.m, the horizontal transverse mode remains as a fundamental mode with good controllability. The thicknesses of the upper and the lower cladding layers both need to be more than 1.5 .mu.m in order to confine light effectively within an active layer in which light generation occurs. In this case, when the width of the lower side of the ridge waveguide 9 is 3 .mu.m, the width of the upper side thereof needs to be less than 1 .mu.m. This results in the upper flat surface of the ridge waveguide 9 becoming so small that it is very difficult to transfer the pattern of an opening thereon in the step of forming the opening 15 of the insulating film 14 on the upper surface of the ridge waveguide 9 in order to make contact with the p side electrode 12. As a result, as shown in FIG. 5, the opening 15 of the insulating film 14 is not correctly positioned on the upper flat surface of the ridge waveguide 9. In this case, the size of the area where the p side electrode 12 is in contact with the p-GaAs contact layer for ohmic contact is effectively reduced, and thus the contact resistance of this area becomes so high that the area may heat and melt during the operation of the device and the operating characteristics will change. A defect could result in the characteristics of the device. When the misposition is so large that the two layers have no contact with each other, it is impossible to fabricate devices and the yield is severely reduced.
Further, when the opening 15 of the insulating film 14 is not correctly positioned on the upper surface of the ridge waveguide 9, the p side electrode 12 does not come into contact with the proper area on the upper surface of the ridge waveguide 9 through which a part of the current flows into the active layer 3. In such a case the current becomes non-uniform and asymmetrical with respect to the ridge waveguide 9. As a result, the current distribution becomes asymmetrical with respect to the ridge waveguide 9, which causes a problem in that the horizontal transverse mode becomes unstable and the level of the light output is lowered because of the occurrence of a kink.
Further, the thermal expansion coefficient of the insulating film is much different from those of semiconductor crystals included in the semiconductor laser. A protruding part like the ridge waveguide 9 is likely to be exposed to stress. Therefore, when the conventional semiconductor laser operates at a high-output, generation of heat during operation causes stress at the junction of the insulating film 14 and semiconductor crystals and crystalline defects are induced at the lower part of the ridge waveguide 9, which reduces the reliability of the semiconductor laser with the passage of time.
Furthermore, the refractive index of 1.5.about.1.9 of the insulating film 14 is much different from those around 3.4 of semiconductor crystals, such as the first upper cladding layer 4, the second upper cladding layer 6, and so on. Therefore, a difference in the refractive indices between the insulating film 14 and semiconductor crystals causes the distribution of the refractive index to become larger along a horizontal transverse direction, namely, a direction perpendicular to the extending direction of the ridge waveguide 9 and parallel to the surface of the substrate 1, whereby higher modes occur easily. A problem occurs whereby the tolerable width of the ridge for producing a fundamental mode becomes so narrow that it becomes very difficult to construct the ridge.